Sources of Signal-Dependent Noise During Isometric Force Production

Abstract

It has been proposed that the invariant kinematics observed during goal-directed movements result from reducing the consequences of signal-dependent noise (SDN) on motor output. The purpose of this study was to investigate the presence of SDN during isometric force production and determine how central and peripheral components contribute to this feature of motor control. Peripheral and central components were distinguished experimentally by comparing voluntary contractions to those elicited by electrical stimulation of the extensor pollicis longus muscle. To determine other factors of motor-unit physiology that may contribute to SDN, a model was constructed and its output compared with the empirical data. SDN was evident in voluntary isometric contractions as a linear scaling of force variability (SD) with respect to the mean force level. However, during electrically stimulated contractions to the same force levels, the variability remained constant over the same range of mean forces. When the subjects were asked to combine voluntary with stimulation-induced contractions, the linear scaling relationship between the SD and mean force returned. The modeling results highlight that much of the basic physiological organization of the motor-unit pool, such as range of twitch amplitudes and range of recruitment thresholds, biases force output to exhibit linearly scaled SDN. This is in contrast to the square root scaling of variability with mean force present in any individual motor-unit of the pool. Orderly recruitment by twitch amplitude was a necessary condition for producing linearly scaled SDN. Surprisingly, the scaling of SDN was independent of the variability of motoneuron firing and therefore by inference, independent of presynaptic noise in the motor command. We conclude that the linear scaling of SDN during voluntary isometric contractions is a natural by-product of the organization of the motor-unit pool that does not depend on signal-dependent noise in the motor command. Synaptic noise in the motor command and common drive, which give rise to the variability and synchronization of motoneuron spiking, determine the magnitude of the force variability at a given level of mean force output.

Experimental muscle pain increases normalized variability of multidirectional forces during isometric contractions

“…This increase of SD with target force is related to the orderly recruitment of motor units: In order to increase muscle force, larger motor units are recruited (Henneman et al 1965), which produce larger and unfused twitches, resulting in increased force fluctuations (Jones et al 2002). As the resultant force represents the spatial summation of activity from different muscles, each contributing to fluctuations on its own direction of action (Kutch et al 2008), an overall increase in muscle activity modulates task-related and tangential forces similarly (Hong et al 2007;Svendsen and Madeleine 2010) , as observed in Fig.…”

“…The SD of force is mainly influenced by the intensity of muscle contraction (Jones et al 2002), and the absence of significant changes in iEMG during pain could explain why no differences in the SD of force were observed across different pain conditions. On the other hand, normalized measures of motor output variability (e.g.…”

Motor variability within a multi-effector system: experimental and analytical studies of multi-finger production of quick force pulses

“…Despite the fact that the errors are relatively small (under 8%), we do not attribute it to "experimental noise" (cf. Jones et al 2002;Van Beers et al 2004). The discrepancy between the model and experimental data is reflected by the term ϑ(t) in Eq.…”

“…Further studies of quick motor actions has led to the emergence of a number of formal models for patterns of motor variability observed during the production of fast discrete movements or force pulses by a single effector (Newell and Carlton 1988;Plamondon and Alimi 1997;Jones et al 2002). The main purpose of the current study has been to develop a model for the production of forces by a redundant motor system based on an earlier model of motor variability during single-joint actions (Gutman and Gottlieb 1992;Gutman et al 1993;Latash and Gutman 1993).…”

Cutaneous mechanisms of isometric ankle force control

“…15 from Rothwell et al 1982). The variability in the force output may come from multiple sources, including variability in the centrally generated motor command and motor noise (Jones et al 2002). This suggests that afferent feedback is necessary to recalibrate force output.…”